Phenylacetylene is present in styrene monomer streams as an undesirable contaminant. Styrene monomer streams, which can include in addition to styrene various substituted styrenes, such as alphamethyl styrene and alkyl ring-substituted styrenes, are formed by the dehydrogenation of the corresponding alkylbenzene, such as ethylbenzene, to form the corresponding vinyl aromatic monomers, such as styrene in the case of ethylbenzene. An undesirable side reaction in such dehydrogenation processes occurs when the ethylbenzene is subjected to a severe dehydrogenation reaction to produce phenylacetylene. While the ethylbenzene, normally common in the resulting styrene product stream, can be readily removed by distillation, the fractional distillation of phenylacetylene and styrene can be accomplished only with difficulty.
In order to provide a purified styrene monomer stream for use in polymerization reactions, it is a conventional practice to selectively hydrogenate the phenylacetylene in the presence of the corresponding styrene monomer. Two types of catalysts may be employed in such phenylacetylene reduction procedures. One type of catalyst, such as disclosed in U.S. Pat. No. 5,156,817 to Butler et al and European Patent Application Publication No. 584,054, also to Butler et al, involves the selective hydrogenation of phenylacetylene over a palladium catalyst supported on an alumina carrier. The palladium catalyst is highly effective and permits the hydrogenation reaction to be carried out under relatively high temperature conditions of about 150° F. and also substantially elevated pressure conditions of about 60–70 psia. Another catalyst used in the selective hydrogenation of phenylacetylene is disclosed in U.S. Pat. No. 4,822,936 to Maurer et al. Here, the catalyst employed is reduced copper on a gamma alumina support. While the Maurer et al process offers the advantage of a catalyst which is less expensive than the palladium catalyst used in the Butler et al process, it also requires relatively modest temperature conditions as well as relatively low pressure conditions. In this respect, while Maurer discloses a hydrogenation temperature below 200° C., preferably in the range of 5° C. to about 100° C., the Maurer procedure is preferably limited to a hydrogenation temperature of less than 35° C. Even at this relatively low temperature, the Maurer procedure requires that the hydrogenation reaction be carried out at ambient or near ambient pressure conditions with a maximum pressure limited to 10 psig, i.e., less than about 25 pounds per square inch absolute.